Carbon dioxide removal from natural gas by membranes in the presence of heavy hydrocarbons and by aqueous diglycolamine®/morpholine
Abstract
Intrinsically defect-free asymmetric hollow fiber polyimide membrane
modules were studied in the presence and absence of saturated and aromatic
components. Results suggest that an essentially defect-free, non-nodular
morphology offers advantages in stability under demanding operating conditions.
Earlier work showed serious losses in performance of membranes comprised of
similar materials, when the selective layer had a pronounced fused nodular nature
as opposed to the intrinsically defect-free skin layers reported on here. Under some
conditions for the ternary system, the permselectivity of the membrane is scarcely
affected, while under other conditions, permselectivity is negatively affected by as
much as 25%. In most cases, for the ternary feeds, significant depression in fluxes
was observed due to competition between the CO2, CH4 and heavier hydrocarbons
but the effect was even more pronounced for the toluene. In addition to steady
state tests in the presence and absence of n-heptane and toluene, modules were
conditioned for five days with ternary mixture of CO2, CH4 and one or the other of
these heavy hydrocarbons. Following this conditioning process, the modules were
studied with a simple binary 10% CO2 /90 % CH4 mixture. These conditioning
studies provide insight into the fundamental effects induced in the membrane due
to the long term exposure to the complex mixtures. Following exposure to the
ternaries containing n-heptane, negligible CO2 permeance increase was seen, while
significantly increased permeances were seen under some conditions following
toluene exposure even at low pressures of the ternary toluene/CO2/CH4
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conditioning gas mixture. Although a more protracted process occurs in the case of
heptane/CO2/CH4 at 35 0C and 500 ppm, a serious loss in selectivity occurs in the
actual ternary tests after exposure for five days. The problem caused by 300 ppm
toluene at 35 0C is more immediately apparent, but the ultimate selectivity loss is
similar. In addition to the selectivity, in the presence of toluene the permeability is
also depressed significantly, presumably due to a greater capability to toluene to
compete for added free volume elements introduced in the conditioning process.
The permeation enhancement due to toluene exposure is lost slowly when the
module downstream is put under vacuum and the gas no longer in contact with the
module for up to three weeks. The conditioning treatment has negligible effect at
55
0C, suggesting that that the sorption affinity of toluene decreases with
increasing temperature. It is seen from the sorption experiments that penetrant
induced conditioning of toluene allows a significant increase in diffusivity than in
solubility coefficients, thus allowing for higher permeability and lower selectivity.
Solubility, rate of absorption and NMR data were obtained for carbon
dioxide in aqueous morpholine (MOR), diglycolamine® (DGA) and aqueous
mixtures of MOR and DGA®. Solubility and rate data were acquired in a wetted
wall contactor. 23.5 wt%, 65 wt% DGA and 11 wt% MOR/53 wt% DGA
concentrations were studied at 298K to 333K. MOR forms an unstable carbamate
upon reaction with CO2 compared to DGA which forms a very stable carbamate.
Morpholine at 11 wt% of the total amine increases the CO2 equilibrium partial
pressure by a factor of 5 to 7 at high loading. The working capacity of 11 wt%
MOR/53 wt% DGA was found to be 10% smaller compared to 65 wt% DGA
under the conditions studied. The heat of reaction of 11 wt% MOR/53 wt%
DGA® was found to be comparable to the 65 wt% DGA. MOR was found also to
be more volatile than DGA. The second order rate constant of DGA was found to
increase linearly with loading by a factor of 5 over a loading range from 0 to 0.4.
Experiments with 65 w% DGA, glycolic acid and potassium formate suggest that
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rate constant increases with loading in the same way as in 65 wt% DGA. The
second order rate constant for MOR (k
25C
2=22000 L/mol s) is four times greater
than DGA (k
25C
2=6600 L/mol s). The MOR reaction with CO2 was found to follow
the zwitterion mechanism; DGA shows zwitterion mechanism in 25 wt% DGA
and second order kinetics in 65 wt% DGA. Predictions made with a rigorous eddy
diffusivity theory suggests that 11 wt% MOR/53 wt% DGA outperforms 65 wt%
DGA of the same concentration by 50 % in terms of CO2 absorption rate. The CO2
enhancement decreases as CO2 loading increases.
Department
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